Abstract
BACKGROUND: IgG4-related disease (IgG4-RD) is a chronic fibro-inflammatory condition that can mimic malignancy due to its tumorous lesions. Marginal zone lymphoma (MZL), a low-grade B cell lymphoma, may occasionally arise in the context of chronic immune stimulation. The synchronous occurrence of IgG4-RD and MZL is rare and diagnostically challenging. CASE REPORT: We report the case of a 39-year-old male with right hemiplegia and aphasia, initially suspected to have a meningioma based on magnetic resonance imaging (MRI) findings. Histopathology following surgical resection revealed features consistent with IgG4-RD. Subsequently, [ (18) F]-FDG PET/CT ([ (18) F]-fluorodeoxyglucose positron emission tomography/computed tomography) demonstrated a metabolically inactive extra-axial supratentorial lesion, as well as a hypermetabolic triangular consolidation in the left lower lung lobe. Lung biopsy confirmed bronchus-associated lymphoid tissue MZL. Immunohistochemistry supported the dual diagnosis. Complementary 99mTc-iFAP SPECT/CT (single photon emission computed tomography/CT) demonstrated focal uptake in the central nervous system lesion but not in the pulmonary lesion. Multimodal image fusion with brain MRI facilitated precise anatomical and functional correlation. DISCUSSION: The coexistence of IgG4-RD and MZL raises important considerations regarding shared immunopathogenic mechanisms. Dual molecular imaging with FDG and FAPI (fibroblast activating protein inhibitor)-based radiotracers proved essential in discriminating between inflammatory and neoplastic components of disease, enabling accurate diagnosis and guiding appropriate management. CONCLUSION: This case highlights the value of [ (18) F]-FDG PET/CT and 99mTc-iFAP SPECT/CT in the evaluation of complex systemic diseases. fibroblast activation protein-targeted imaging may play a key role in the diagnostic workup of IgG4-RD and in distinguishing it from coexisting lymphoproliferative disorders.